Dynamic spine stabilizer

ABSTRACT

A dynamic spine stabilizer moves under the control of spinal motion providing increased mechanical support within a central zone corresponding substantially to the neutral zone of the injured spine. The dynamic spine stabilizer includes a support assembly and a resistance assembly associated with the support assembly. The resistance assembly generates greater increase in mechanical force during movement within the central zone and lesser increase in mechanical force during movement beyond the central zone. A method for using the stabilizer is also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of U.S.Provisional Application Ser. No. 60/506,724, entitled “DYNAMIC SPINESTABILIZER”, filed Sep. 30, 2003, and U.S. Provisional PatentApplication Ser. No. 60/467,414, entitled “DYNAMIC SPINE STABILIZER”,filed May 2, 2003. This application is a continuation-in-part of U.S.Non-Provisional Application Ser. No. 10/835,109, entitled “DYNAMIC SPINESTABILIZER”, filed Apr. 30, 2004 now U.S. Pat. No. 7,029,475.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method and apparatus for spinalstabilization. More particularly, the invention relates to a method andapparatus for applying increased incremental mechanical resistance whenthe spine moves within its neutral zone.

2. Description of the Prior Art

Low back pain is one of the most expensive diseases afflictingindustrialized societies. With the exception of the common cold, itaccounts for more doctor visits than any other ailment. The spectrum oflow back pain is wide, ranging from periods of intense disabling painwhich resolve, to varying degrees of chronic pain. The conservativetreatments available for lower back pain include: cold packs, physicaltherapy, narcotics, steroids and chiropractic maneuvers. Once a patienthas exhausted all conservative therapy, the surgical options range frommicro discectomy, a relatively minor procedure to relieve pressure onthe nerve root and spinal cord, to fusion, which takes away spinalmotion at the level of pain.

Each year, over 200,000 patients undergo lumbar fusion surgery in theUnited States. While fusion is effective about seventy percent of thetime, there are consequences even to these successful procedures,including a reduced range of motion and an increased load transfer toadjacent levels of the spine, which accelerates degeneration at thoselevels. Further, a significant number of back-pain patients, estimatedto exceed seven million in the U.S., simply endure chronic low-backpain, rather than risk procedures that may not be appropriate oreffective in alleviating their symptoms.

New treatment modalities, collectively called motion preservationdevices, are currently being developed to address these limitations.Some promising therapies are in the form of nucleus, disc or facetreplacements. Other motion preservation devices provide dynamic internalstabilization of the injured and/or degenerated spine, without removingany spinal tissues. A major goal of this concept is the stabilization ofthe spine to prevent pain while preserving near normal spinal function.The primary difference in the two types of motion preservation devicesis that replacement devices are utilized with the goal of replacingdegenerated anatomical structures which facilitates motion while dynamicinternal stabilization devices are utilized with the goal of stabilizingand controlling abnormal spinal motion.

Over ten years ago a hypothesis of low back pain was presented in whichthe spinal system was conceptualized as consisting of the spinal column(vertebrae, discs and ligaments), the muscles surrounding the spinalcolumn, and a neuromuscular control unit which helps stabilize the spineduring various activities of daily living. Panjabi M M. “The stabilizingsystem of the spine. Part I. Function, dysfunction, adaptation, andenhancement.” J Spinal Disord 5 (4): 383-389, 1992a. A corollary of thishypothesis was that strong spinal muscles are needed when a spine isinjured or degenerated. This was especially true white standing inneutral posture. Panjabi M M. “The stabilizing system of the spine. PartII. Neutral zone and instability hypothesis.” J Spinal Disord 5 (4):390-397, 1992b. In other words, a low-back patient needs to havesufficient well-coordinated muscle forces, strengthening and trainingthe muscles where necessary, so they provide maximum protection whilestanding in neutral posture.

Dynamic stabilization (non-fusion) devices need certain functionality inorder to assist the compromised (injured or degenerated with diminishedmechanical integrity) spine of a back patient. Specifically, the devicesmust provide mechanical assistance to the compromised spine, especiallyin the neutral zone where it is needed most. The “neutral zone” refersto a region of low spinal stiffness or the toe-region of theMoment-Rotation curve of the spinal segment (see FIG. 1). Panjabi M M,Goel V K, Takata K. 1981 Volvo Award in Biomechanics. “PhysiologicalStrains in Lumbar Spinal Ligaments, an in vitro Biomechanical Study.”Spine 7 (3): 192-203, 1982. The neutral zone is commonly defined as thecentral part of the range of motion around the neutral posture where thesoft tissues of the spine and the facet joints provide least resistanceto spinal motion. This concept is nicely visualized on aload-displacement or moment-rotation curve of an intact and injuredspine as shown in FIG. 1. Notice that the curves are non-linear; thatis, the spine mechanical properties change with the amount ofangulations and/or rotation. If we consider curves on the positive andnegative sides to represent spinal behavior in flexion and extensionrespectively, then the slope of the curve at each point representsspinal stiffness. As seen in FIG. 1, the neutral zone is the lowstiffness region of the range of motion.

Experiments have shown that after an injury of the spinal column or dueto degeneration, neutral zones, as well as ranges of motion, increase(see FIG. 1). However, the neutral zone increases to a greater extentthan does the range of motion, when described as a percentage of thecorresponding intact values. This implies that the neutral zone is abetter measure of spinal injury and instability than the range ofmotion. Clinical studies have also found that the range of motionincrease does not correlate well with low back pain. Therefore, theunstable spine needs to be stabilized especially in the neutral zone.Dynamic internal stabilization devices must be flexible so as to movewith the spine, thus allowing the disc, the facet joints, and theligaments normal physiological motion and loads necessary formaintaining their nutritional well-being. The devices must alsoaccommodate the different physical characteristics of individualpatients and anatomies to achieve a desired posture for each individualpatient.

With the foregoing in mind, those skilled in the art will understandthat a need exists for a spinal stabilization device which overcomes theshortcoming of prior art devices. The present invention provides such anapparatus and method for spinal stabilization.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a methodfor spinal stabilization. The method is achieved by securing a dynamicstabilizer to vertebrae of a spine and providing mechanical assistancein the form of resistance to a region of the spine to which the dynamicstabilizer is attached. The resistance is applied such that greatermechanical assistance is provided while the spine is around its neutralzone and lesser mechanical assistance is provided while the spine bendsbeyond its neutral zone.

It is also an object of the present invention to provide a dynamicstabilizer that moves under the control of spinal motion providingincreased mechanical support within a central zone correspondingsubstantially to a neutral zone of an injured spine. The stabilizerincludes a support assembly and a resistance assembly associated withthe support assembly. The resistance assembly generates resistanceapplying greater resistance to movement during movement within thecentral zone and lower resistance to movement while the stabilizerundergoes extended movement beyond its central zone.

It is another object of the present invention to provide a dynamicstabilizer including a piston assembly and a resistance assemblyassociated with the piston assembly. The resistance assembly is composedof a first spring and a second spring and the piston assembly is shapedand dimensioned or linking the resistance assembly to a boded member.

Other objects and advantages of the present invention will becomeapparent from the following detailed description when viewed inconjunction with the accompanying drawings, which set forth certainembodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is Moment-Rotation curve for a spinal segment (intact andinjured), showing the low spinal stiffness within the neutral zone.

FIG. 2 is a schematic representation of a spinal segment in conjunctionwith a Moment-Rotation curve for a spinal segment, showing the lowspinal stiffness within the neutral zone.

FIG. 3 a is a schematic of the present invention in conjunction with aForce-Displacement curve, demonstrating the increased resistanceprovided within the central zone of the present dynamic spinestabilizer.

FIG. 3 b is a Force-Displacement curve demonstrating the change inprofile achieved through the replacement of springs.

FIG. 3 c is a dorsal view of the spine with a pair of stabilizerssecured thereto.

FIG. 3 d is a side view showing the stabilizer in tension.

FIG. 3 e is a side view showing the stabilizer in compression.

FIG. 4 is a schematic of the present dynamic spine stabilizer.

FIG. 5 is a schematic of an alternate embodiment in accordance with thepresent invention.

FIG. 6 is a Moment-Rotation curve demonstrating the manner in which thepresent stabilizer assists spinal stabilization.

FIGS. 7 a and 7 b are respectively a free body diagram of the presentstabilizer and a diagram representing the central zone of the presentstabilizer.

FIG. 8 is a bar graph reflecting flexion/extension data based on cadaverstudies that included an exemplary dynamic stabilization deviceaccording to the present disclosure.

FIG. 9 is a bar graph reflecting lateral bending data based on cadaverstudies that included an exemplary dynamic stabilization deviceaccording to the present disclosure.

FIG. 10 is a bar graph reflecting axial rotation data based on cadaverstudies that included an exemplary dynamic stabilization deviceaccording to the present disclosure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The detailed embodiments of the present invention are disclosed herein.It should be understood, however, that the disclosed embodiments aremerely exemplary of the invention, which may be embodied in variousforms. Therefore, the details disclosed herein are not to be interpretedas limited, but merely as the basis for the claims and as a basis forteaching one skilled in the art how to make and/or use the invention.

With reference to FIGS. 2, 3 a-c and 4, a method and apparatus aredisclosed for spinal stabilization. In accordance, with a preferredembodiment of the present invention, the spinal stabilization method isachieved by securing an internal dynamic spine stabilizer 10 betweenadjacent vertebrae 12, 14 and providing mechanical assistance in theform of elastic resistance to the region of the spine to which thedynamic spine stabilizer 10 is attached. The elastic resistance isapplied as a function of displacement such that greater mechanicalassistance is provided while the spine is in its neutral zone and lessermechanical assistance is provided while the spine bends beyond itsneutral zone. Although the term elastic resistance is used throughoutthe body of the present specification, other forms of resistance may beemployed without departing from the spirit of the present invention.

As those skilled in the art will certainly appreciate, and as mentionedabove, the “neutral zone” is understood to refer to a region of lowspinal stiffness or the toe-region of the Moment-Rotation curve of thespinal segment (see FIG. 2). That is, the neutral zone may be consideredto refer to a region of laxity around the neutral resting position of aspinal segment where there is minimal resistance to intervertebralmotion. The range of the neutral zone is considered to be of majorsignificance in determining spinal stability. Panjabi, M M. “Thestabilizing system of the spine. Part II. Neutral zone and instabilityhypothesis.” J Spinal Disorders 1992; 5(4): 390-397.

In fact, the inventor has previously described the load displacementcurve associated with spinal stability through the use of a “ball in abowl” analogy. According to this analogy, the shape of the bowlindicates spinal stability. A deeper bowl represents a more stablespine, while a more shallow bowl represents a less stable spine. Theinventor previously hypothesized that for someone without spinal injurythere is a normal neutral zone (that part of the range of motion wherethere is minimal resistance to intervertebral motion) with a normalrange of motion, and in turn, no spinal pain. In this instance, the bowlis not too deep nor too shallow. However, when an injury occurs to ananatomical structure, the neutral zone of the spinal column increasesand the ball moves freely over a larger distance. By this analogy, thebowl would be more shallow and the ball less stable, and consequently,pain results from this enlarged neutral zone.

In general, pedicle screws 16, 18 attach the dynamic spine stabilizer 10to the vertebrae 12, 14 of the spine using well-tolerated and familiarsurgical procedures known to those skilled in the art. In accordancewith a preferred embodiment, and as those skilled in the art willcertainly appreciate, a pair of opposed stabilizers are commonly used tobalance the loads applied to the spine (see FIG. 3 c). The dynamic spinestabilizer 10 assists the compromised (injured and/or degenerated) spineof a back pain patient, and helps her/him perform daily activities. Thedynamic spine stabilizer 10 does so by providing controlled resistanceto spinal motion particularly around neutral posture in the region ofneutral zone. As the spine bends forward (flexion) the stabilizer 10 istensioned (see FIG. 3 d) and when the spine bends backward (extension)the stabilizer 10 is compressed (see FIG. 3 e).

The resistance to displacement provided by the dynamic spine stabilizer10 is non-linear, being greatest in its central zone so as to correspondto the individual's neutral zone; that is, the central zone of thestabilizer 10 provides a high level of mechanical assistance insupporting the spine. As the individual moves beyond the neutral zone,the increase in resistance decreases to a more moderate level. As aresult, the individual encounters greater resistance to movement (orgreater incremental resistance) while moving within the neutral zone.

The central zone of the dynamic spine stabilizer 10, that is, the rangeof motion in which the spine stabilizer 10 provides the greatestresistance to movement, is adjustable at the time of surgery to suit theneutral zone of each individual patient. The resistance to movementprovided by the dynamic spine stabilizer 10 is adjustablepre-operatively and intra-operatively. This helps to tailor themechanical properties of the dynamic spine stabilizer 10 to suit thecompromised spine of the individual patient. The length of the dynamicspine stabilizer 10 is also adjustable intra-operatively, to suitindividual patient anatomy and to achieve desired spinal posture. Thedynamic spine stabilizer 10 can be re-adjusted post-operatively with asurgical procedure to adjust its central zone to accommodate a patient'saltered needs.

Ball joints 36, 38 link the dynamic spine stabilizer 10 with the pediclescrews 16, 18. The junction of the dynamic spine stabilizer 10 andpedicle screws 16, 18 is free and rotationally unconstrained. Therefore,first of all, the spine is allowed all physiological motions of bendingand twisting and second, the dynamic spine stabilizer 10 and the pediclescrews 16, 18 are protected from harmful bending and torsional forces,or moments. While ball joints are disclosed in accordance with apreferred embodiment of the present invention, other linking structuresmay be utilized without departing from the spirit of the presentinvention.

As there are ball joints 36, 38 at each end of the stabilizer 10, nobending moments can be transferred from the spine to the stabilizer 10.Further, it is important to recognize the only forces that act on thestabilizer 10 are those due to the forces of the springs 30, 32 withinit. These forces are solely dependent upon the tension and compressionof the stabilizer 10 as determined by the spinal motion. In summary, thestabilizer 10 sees only the spring forces. Irrespective of the largeloads on the spine, such as when a person carries or lifts a heavy load,the loads coming to the stabilizer 10 are only the forces developedwithin the stabilizer 10, which are the result of spinal motion and notthe result of the spinal load. The stabilizer 10 is, therefore, uniquelyable to assist the spine without enduring the high loads of the spine,allowing a wide range of design options.

The loading of the pedicle screws 16, 18 in the present stabilizer 10 isalso quite different from that in prior art pedicle screw fixationdevices. The only load the stabilizer pedicle screws 16, 18 see is theforce from the stabilizer 10. This translates into pure axial force atthe ball joint-screw interface. This mechanism greatly reduces thebending moment placed onto the pedicle screws 16, 18 as compared toprior art pedicle screw fusion systems. Due to the ball joints 36, 38,the bending moment within the pedicle screws 16, 18 is zero at the balljoints 36, 38 and it increases toward the tip of the pedicle screws 16,18. The area of pedicle screw-bone interface which often is the failuresite in a typical prior art pedicle screw fixation device, is a lessstressed site relative to prior art implementations, and is thereforenot likely to fail. In sum, the pedicle screws 16, 18, when used inconjunction with the present invention, carry significantly less loadand are placed under significantly less stress than typical pediclescrews.

In FIG. 2, the Moment-Rotation curve for a healthy spine is shown inconfigurations with the present stabilizer 10. This curve shows the lowresistance to movement encountered in the neutral zone of a healthyspine. However, when the spine is injured, this curve changes and thespine becomes unstable, as evidenced by the expansion of the neutralzone (see FIG. 1).

In accordance with a preferred embodiment of the present invention,people suffering from spinal injuries are best treated through theapplication of increased mechanical assistance in the neutral zone. Asthe spine moves beyond the neutral zone, the necessary mechanicalassistance decreases and becomes more moderate. In particular, and withreference to FIG. 3 a, the support profile contemplated in accordancewith the present invention is disclosed.

Three different profiles are shown in FIG. 3 a. The disclosed profilesare merely exemplary and demonstrate the possible support requirementswithin the neutral zone. Profile 1 is exemplary of an individualrequiring great assistance in the neutral zone and the central zone ofthe stabilizer is therefore increased providing a high level ofresistance over a great displacement; Profile 2 is exemplary of anindividual where less assistance is required in the neutral zone and thecentral zone of the stabilizer is therefore more moderate providingincreased resistance over a more limited range of displacement; andProfile 3 is exemplary of situations where only slightly greaterassistance is required in the neutral zone and the central zone of thestabilizer may therefore be decreased to provide increased resistanceover even a smatter range of displacement.

As those skilled in the art will certainly appreciate, the mechanicalassistance required and the range of the neutral zone will vary fromindividual to individual. However, the basic tenet of the presentinvention remains; that is, greater mechanical assistance for thoseindividuals suffering from spinal instability is required within theindividual's neutral zone. This assistance is provided in the form ofgreater resistance to movement provided within the neutral zone of theindividual and the central zone of the dynamic spine stabilizer 10.

The dynamic spine stabilizer 10 developed in accordance with the presentinvention provides mechanical assistance in accordance with thedisclosed support profile. Further, the present stabilizer 10 providesfor adjustability via a concentric spring design.

More specifically, the dynamic spine stabilizer 10 provides assistanceto the compromised spine in the form of increased resistance to movement(provided by springs in accordance, with a preferred embodiment) as thespine moves from the neutral posture, in any physiological direction. Asmentioned above, the Force-Displacement relationship provided by thedynamic spine stabilizer 10 in accordance with the present invention isnon-linear, with greater incremental resistance around the neutral zoneof the spine and central zone of the stabilizer 10, and decreasingincremental resistance beyond the central zone of the dynamic spinestabilizer 10 as the individual moves beyond the neutral zone (see FIG.3 a).

The relationship of the present stabilizer 10 to forces applied duringtension and compression is further shown with reference to FIG. 3 a. Asdiscussed above, the behavior of the present stabilizer 10 isnon-linear. The Load-Displacement curve has three zones: tension,central and compression. If K1 and K2 define the stiffness values in thetension and compression zones respectively, the present stabilizer isdesigned such that the high stiffness in the central zone is “K1+K2”.Depending upon the preload of the stabilizer 10 as will be discussedbelow in greater detail, the width of the central zone and, therefore,the region of high stiffness can be adjusted.

With reference to FIG. 4, a dynamic spine stabilizer 10 in accordancewith the present invention is disclosed. The dynamic spine stabilizer 10includes a support assembly in the form of a housing 20 composed of afirst housing member 22 and a second housing member 24. The firsthousing member 22 and the second housing member 24 are telescopicallyconnected via external threads formed upon the open end 26 of the firsthousing member 22 and internal threads formed upon the open end 28 ofthe second housing member 24. In this way, the housing 20 is completedby screwing the first housing member 22 into the second housing member24. As such, and as will be discussed below in greater detail, therelative distance between the first housing member 22 and the secondhousing member 24 can be readily adjusted for the purpose of adjustingthe compression of the first spring 30 and second spring 32 containedwithin the housing 20. Although springs are employed in accordance witha preferred embodiment of the present invention, other elastic membersmay be employed without departing from the spirit of the presentinvention. Ad piston assembly 34 links the first spring 30 and thesecond spring 32 to first and second ball joints 36, 38. The first andsecond ball joints 36, 38 are in turn shaped and designed for selectiveattachment to pedicle screws 16, 18 extending from the respectivevertebrae 12, 14.

The first ball joint 36 is secured to the closed end 39 of the firsthousing member 22 via a threaded engagement member 40 shaped anddimensioned for coupling, with threads formed within an aperture 42formed in the closed end 39 of the first housing member 22. In this way,the first ball joint 36 substantially closes off the closed end 39 ofthe first housing member 22. The length of the dynamic spine stabilizer10 may be readily adjusted by rotating the first ball joint 36 to adjustthe extent of overlap between the first housing member 22 and theengagement member 40 of the first ball joint 36. As those skilled in theart will certainly appreciate, a threaded engagement between the firsthousing member 22 and the engagement member 40 of the first ball joint36 is disclosed in accordance with a preferred embodiment, althoughother coupling structures may be employed without departing from thespirit of the present invention.

The closed end 44 of the second housing member 24 is provided with a cap46 having an aperture 48 formed therein. As will be discussed below ingreater detail, the aperture 48 is shaped and dimensioned for thepassage of a piston rod 50 from the piston assembly 34 therethrough.

The piston assembly 34 includes a piston rod 50; and retaining rods 52that cooperate with first and second springs 30, 32. The piston rod 50includes a stop nut 54 and an enlarged head 56 at its first end 58. Theenlarged head 56 is rigidly connected to the piston rod 50 and includesguide holes 60 through which the retaining rods 52 extend duringoperation of the present dynamic spine stabilizer 10. As such, theenlarged head 56 is guided along the retaining rods 52 while the secondball joint 38 is moved toward and away from the first ball joint 36. Aswill be discussed below in greater detail, the enlarged head 56interacts with the first spring 30 to create resistance as the dynamicspine stabilizer 10 is extended and the spine is moved in flexion.

A stop nut 54 is fit over the piston rod 50 for free movement relativethereto. However, movement of the stop nut 54 toward the first balljoint 36 is prevented by the retaining rods 52 that support the stop nut54 and prevent the stop nut 54 from moving toward the first ball joint36. As will be discussed below in greater detail, the stop nut 54interacts with the second spring 32 to create resistance as the dynamicspine stabilizer 10 is compressed and the spine is moved in extension.

The second end 62 of the piston rod 50 extends from the aperture 48 atthe closed end 44 of the second housing member 24, and is attached to anengagement member 64 of the second ball joint 38. The second end 62 ofthe piston rod 50 is coupled to the engagement member 64 of the secondball joint 38 via a threaded engagement. As those skilled in the artwill certainly appreciate, a threaded engagement between the second end62 of the piston rod 50 and the engagement member 64 of the second balljoint 38 is disclosed in accordance with a preferred embodiment,although other coupling structures may be employed without departingfrom the spirit of the present invention.

As briefly mentioned above, the first and second springs 30, 32 are heldwithin the housing 20. In particular, the first spring 30 extendsbetween the enlarged head 56 of the piston rod 50 and the cap 46 of thesecond housing member 24. The second spring 32 extends between thedistal end of the engagement member 64 of the second ball joint 38 andthe stop nut 54 of the piston rod 50. The preloaded force applied by thefirst and second springs 30, 32 holds the piston rod in a staticposition within the housing, 20, such that the piston rod is able tomove during either extension or flexion of the spine.

In use, when the vertebrae 12, 14 are moved in flexion and the firstball joint 36 is drawn away from the second ball joint 38, the pistonrod 50 is pulled within the housing 24 against the force being appliedby the first spring 30. In particular, the enlarged head 56 of thepiston rod 50 is moved toward the closed end 44 of the second housingmember 24. This movement causes compression of the first spring 30,creating resistance to the movement of the spine. With regard to thesecond spring 32, the second spring 32, which is captured between stopnut 54 and second ball joint 38, extends or lengthens as a result ofmovement of second ball joint 38 away from first ball joint 36. As thevertebrae move in flexion within the neutral zone, the height of thesecond spring 32 is increased, reducing the distractive force, and ineffect increasing the resistance of the device to movement. Through thismechanism, as the spine moves in flexion from the initial position bothspring 30 and spring 32 resist the distraction of the device directly,either by increasing the load within the spring (i.e. first spring 30)or by decreasing the toad assisting the motion (i.e. second spring 32).

However, when the spine is in extension, and the second ball joint 38 ismoved toward the first ball joint 36, the engagement member 64 of thesecond ball joint 38 moves toward the stop nut 54, which is held isplace by the retaining rods 52 as the piston rod 50 moves toward thefirst ball joint 36. This movement causes compression of the secondspring 32 held between the engagement member 64 of the second ball joint38 and the stop nut 54, to create resistance to the movement of thedynamic spine stabilizer 10. With regard to the first spring 30, thefirst spring 30 is supported between the cap 46 and the enlarged head56, and as the vertebrae move in extension within the neutral zone, theheight of the second spring 30 is increased, reducing the compressiveforce, and in effect increasing the resistance of the device tomovement. Through this mechanism, as the spine moves in extension fromthe initial position both spring 32 and spring 30 resist the compressionof the device directly, either by increasing the load within the spring(i.e. second spring 32) or by decreasing the load assisting the motion(i.e. first spring 30).

Based upon the use of two concentrically positioned elastic springs 30,32 as disclosed in accordance with the present invention, an assistance(force) profile as shown in FIG. 2 is provided by the present dynamicspine stabilizer 10. That is, the first and second springs 30, 32 workin conjunction to provide a large elastic force when the dynamic spinestabilizer 10 is displaced within the central zone of the stabilizer.However, once displacement between the first ball joint 36 and thesecond ball joint 38 extends beyond the central zone of the stabilizer10 and the neutral zone of the individual's spinal movement, tileincremental resistance to motion is substantially reduced as theindividual no longer requires the substantial assistance needed withinthe neutral zone. This is accomplished by setting the central zone ofthe device disclosed herein. The central zone of the force displacementcurve is the area of the curve which represents when both springs areacting in the device as described above. When the motion of the spine isoutside the neutral zone and the correlating device elongation orcompression is outside the set central zone, the spring which iselongating reaches its free length. Free length, as anybody skilled inthe art will appreciate, is the length of a spring when no force isapplied. In this mechanism the resistance to movement of the deviceoutside the central zone (where both springs are acting to resistmotion) is only reliant on the resistance of one spring: either spring30 in flexion or spring 32 in extension.

As briefly discussed above, the dynamic spine stabilizer 10 is adjustedby rotation of the first housing member 22 relative to the secondhousing member 24. This movement changes the distance between the firsthousing member 22 and the second housing member 24 in a manner whichultimately changes the preload placed across the first and secondsprings 30, 32. This change in preload alters the resistance profile ofthe present dynamic spine stabilizer 10 from that shown in Profile 2 ofFIG. 3 a to an increase in preload (see Profile 1 of FIG. 3 a) whichenlarges the effective range in which the first and second springs 30,32 act in unison. This increased width of the central zone of thestabilizer 10 correlates to higher stiffness over a larger range ofmotion of the spine. This effect can be reversed as evident in Profile 3of FIG. 3 a.

The present dynamic spine stabilizer 10 is attached to pedicle screws16, 18 extending from the vertebral section requiring support. Duringsurgical attachment of the dynamic spine stabilizer 10, the magnitude ofthe stabilizer's central zone can be adjusted for each individualpatient, as judged by the surgeon and/or quantified by an instabilitymeasurement device. This adjustable feature of the dynamic spinestabilizer 10 is exemplified in the three explanatory profiles that havebeen generated in accordance with a preferred embodiment of the presentinvention (see FIG. 2; note the width of the device central zones).

Pre-operatively, the first and second elastic springs 30, 32 of thedynamic spine stabilizer 10 can be replaced by a different set toaccommodate a wider range of spinal instabilities. As expressed in FIG.3 b, Profile 2 b demonstrates the force displacement curve generatedwith a stiffer set of springs when compared with the curve shown inProfile 2 a of FIG. 3 b.

Intra-operatively, the length of the dynamic spine stabilizer 10 isadjustable by turning the engagement member 40 of the first ball joint36 to lengthen the stabilizer 10 in order to accommodate differentpatient anatomies and desired spinal posture. Pre-operatively, thepiston rod 50 may be replaced to accommodate an even wider range ofanatomic variation.

The present dynamic spine stabilizer 10 has been tested alone for itsload-displacement relationship. When applying tension, the dynamic spinestabilizer 10 demonstrated increasing resistance up to a pre-defineddisplacement, followed by a reduced rate of increasing resistance untilthe device reached its fully elongated position. When subjected tocompression, the dynamic spine stabilizer 10 demonstrated increasingresistance up to a pre-defined displacement, followed by a reduced rateof increasing resistance until the device reached its fully compressedposition. Therefore, the dynamic spine stabilizer 10 exhibits aload-displacement curve that is non-linear with the greatest resistanceto displacement offered around the neutral posture. This behavior helpsto normalize the load-displacement curve of a compromised spine.

In another embodiment of the design, with reference to FIG. 5, thestabilizer 110 may be constructed with an in-line spring arrangement. Inaccordance with this embodiment, the housing 120 is composed of firstand second housing members 122, 124 which are coupled with threadsallowing for adjustability. A first ball joint 136 extends from thefirst housing member 122. The second housing member 124 is provided withan aperture 148 through which the second end 162 of piston rod 150extends. The second end 162 of the piston rod 150 is attached to thesecond ball joint 138. The second ball joint 138 is screwed onto thepiston rod 150.

The piston rod 150 includes an enlarged head 156 at its first end 158.The first and second springs 130, 132 are respectively secured betweenthe enlarged head 156 and the closed ends 139, 144 of the first andsecond housing members 122, 124. In this way, the stabilizer 110provides resistance to both expansion and compression using the samemechanical principles described for the previous embodiment.

Adjustment of the resistance profile in accordance with this alternateembodiment is achieved by rotating the first housing member 122 relativeto the second housing member 124. Rotation in this way alters thecentral zone of high resistance provided by the stabilizer 110. Aspreviously described one or both springs may also be exchanged to changethe slope of the force-displacement curve in two or three zonesrespectively.

To explain how the stabilizer 10, 110 assists a compromised spine(increased neutral zone) observe the moment-rotation curves (FIG. 6).Four curves are shown: 1. Intact, 2. Injured, 3. Stabilizer and, 4.Injured+Stabilizer. These are, respectively, the Moment-Rotation curvesof the intact spine, injured spine, stabilizer alone, and stabilizerplus injured spine Notice that this curve is close to the intact curve.Thus, the stabilizer, which provides greater resistance to movementaround the neutral posture, is ideally suited to compensate for theinstability of the spine.

In addition to the dynamic spine stabilizer described above, othercomplementary devices are contemplated. For example, a link-device maybe provided for joining the left- and right-stabilizer units to helpprovide additional stability in axial rotation and lateral bending. Thislink-device will be a supplement to the dynamic spine stabilizer. Itwill be applied as needed on an individual patient basis. In addition, aspinal stability measurement device may be utilized. The measurementdevice will quantify the stability of each spinal level at the time ofsurgery. This device will attach intra-operatively to a pair of adjacentspinal components at compromised and uncompromised spinal levels tomeasure the stability of each level. The stability measurements of theadjacent uninjured levels relative to the injured level(s) can be usedto determine the appropriate adjustment of the device. Additionally, thestability measurements of the injured spinal level(s) can be used toadjust the device by referring to a tabulated database of normaluninjured spinal stabilities. The device will be simple and robust, sothat the surgeon is provided with the information in the simplestpossible manner under operative conditions.

The choice of spring used in accordance with the present invention toachieve the desired force profile curve is governed by the basicphysical laws governing the force produced by springs. In particular,the force profile described above and shown in FIG. 3 a is achievedthrough the unique design of the present stabilizer.

First, the stabilizer functions both in compression and tension, eventhrough the two springs within the stabilizer are both of compressiontype. Second, the higher stiffness (K₁+K₂) provided by the stabilizer inthe central zone is due to the presence of a preload. Both springs aremade to work together, when the preload is present. As the stabilizer iseither tensioned or compressed, the force increases in one spring anddecreases in the other. When the decreasing force reaches the zerovalue, the spring corresponding to this force no longer functions, thusdecreasing the stabilizer function, an engineering analysis, includingthe diagrams shown in FIGS. 7 a and 7 b, is presented below (theanalysis specifically relates to the embodiment disclosed in FIG. 5,although those skilled in the art will appreciate the way in which itapplies to all embodiments disclosed in accordance with the presentinvention).

-   -   F₀ is the preload within the stabilizer, introduced by        shortening the body length of the housing as discussed above.    -   K₁ and K₂ are stiffness coefficients of the compression springs,        active during stabilizer tensioning and compression,        respectively.    -   F and D are respectively the force and displacement of the disc        of the stabilizer with respect to the body of the stabilizer.    -   The sum of forces on the disc must equal zero. Therefore,        F+(F ₀ −D×K ₂)−(F ₀ +D×K ₁)=0, and        F=D×(K ₁ +K ₂).    -   With regard to the central zone (CZ) width (see FIG. 3 a):        -   On Tension side CZ_(T) is:            CZ _(T) =F ₀ /K ₂.        -   On Compression side CZ_(T) is:            CZ _(c) =F ₀ /K ₁.

EXPERIMENAL RESULTS

To evaluate a stabilization device according to the present disclosure,cadaver response to applied moments in predetermined modalities wastested. In particular, measurements were made with respect to range ofmotion (ROM), neutral zone (NZ) and a high flexibility zone (HFZ). Theexperimental study was undertaken to determine whether a stabilizationdevice according to the present disclosure is effective in reducingspinal instability (measured as a reduction in NZ and HFZ), whileallowing normal ROM.

Study Design and Setting: The characteristics of five (5) cadavericmotion segments were evaluated in five (5) states: (i) intact; (ii)nucleotomy (N); (iii) nucleotomy plus stabilization device; (iv)laminectomy with partial facetectomy (LPF); and (v) LPF plusstabilization device. Each injury was chosen based on its history of useand clinical significance. Five human lumbar cadaver specimens wereused, namely four L3-4 segments and one L1-2 segment.

Methods: Specimens were obtained within 24 hours of death and stored insaline soaked gauze at −20° C. until the time of testing. The specimenswere thawed and extraneous tissue removed. Plain radiographs were takenof the spines to determine anatomy, degree of disc degeneration andpre-existing bony pathology (if any). Specimens with pathology (e.g.,bridging osteophytes, Schmol's nodes or obvious facet degeneration) wereexcluded from the study. Specimens with significant pre-existing discpathology (such as herniation) were also excluded from the study.

Pedicle screws were placed bilaterally in the inferior and superiorvertebral bodies. Additional augmentation of pedicle screw fixation wasachieved by removing the pedicle screw, adding a small amount of epoxy(≈1 cc), and reinserting the screw. Pedicle screws were wrapped insaline soaked paper and each motion segment was potted in low meltingtemperature alloy. The construct was placed in test equipment adapted toprovide multiple degrees of freedom. The potting fixture was bolted tothe testing machine such that the specimen was rigidly attached relativeto the machine. The inferior fixture rested on an x-y table whichallowed the specimen unconstrained free motion during testing.

A six-axis load cell (AMFI, Inc., Watertown, Mass.) was used to measurethe forces and torques applied to the specimen during testing. An axialcompressive load was applied continuously to the specimen (preload of200N), while pure bending moments in flexion/extension, left/rightlateral bending, and left/right torsion were applied to the superiorvertebral body of the specimen. Relative changes in position andangulation were measured with high-resolution optical encoders (GurleyPrecision Instruments, Troy, N.Y.). Displacement of the stabilizationdevice between the pedicle screws was measured using two positiontransducers (SpaceAge Control, Palmdale, Calif.). Data were collected ata minimum sampling rate of 10 Hz.

Intact specimens (no injury and no stabilization) were loaded throughthree cycles each to 10 Nm in flexion/extension, left/right lateralbending, and left/right torsion at 1 mm/minute with a continuous 200 Naxial compressive preload. Following completion of intact testing,specimens were removed from the test machine. Following placement of thestabilization device/system of the present disclosure, specimens wereplaced back into the test machine and the test protocol was repeated. Inthe tests described herein, a stabilization device of the type depictedin FIG. 5. Each motion segment was again loaded through 3 cycles offorward flexion/extension, left/right lateral bending, and left/righttorsion under a continuous compressive 200 N axial compressive pre-load.Testing was repeated under the following conditions: (i) nucleotomy withno stabilization, nucleotomy with stabilization, laminectomy withpartial facetectomy (LPF) with no stabilization, and LPF withstabilization.

Outcome Measures: Following the completion of the testing, raw data textfiles were exported to a Microsoft Excel program. Data included cyclenumber, motion, current angle, current moment, axial load, displacementtransducer on right side, and displacement transducer on left side.Range of motion at 10 Nm, neutral zone at 2.5 Nm (active curve), neutralzone at 0.2 Nm (passive curve), and displacement of the pedicle screwsof the uninstrumented constructs (i.e., in the absence of a dynamicstabilization device according to the present disclosure) were comparedto the instrumented constructs (i.e., with a dynamic stabilizationdevice according to the present disclosure). ROM, NZ and HFZ werereported for Flexion/Extension, Lateral Bending and Axial Rotation.ROM=rotation±10N-m; NZ=rotation±0.2 Nm of the passive response prior tocrossing the zero moment axis; JFZ=rotation±2.5 Nm on the active curve.

Results: Due to specimen degradation, two (2) specimens were notevaluated in LPF and LPF plus stabilization device. Mean range ofmotion, neutral zone and displacement data for each construct inflexion/extension, lateral bending, and axial rotation are set forth inthe bar graphs of FIGS. 8-10. As the bar graphs show, spinal instabilityincreases with surgical injury. This may be measured as an increase inROM and a significantly higher relative increase in NZ and HFZ. Throughuse of the disclosed stabilization device as described herein, it waspossible to advantageously reduce NZ and HFZ to levels that arecomparable to intact levels, while simultaneously leaving ROMuncompromised.

As those skilled in the art will certainly appreciate, the conceptsunderlying the present invention may be applied to other medicalprocedures. As such, these concepts may be utilized beyond spinaltreatments without departing from the spirit of the present invention.

While the preferred embodiments have been shown and described, it willbe understood that there is no intent to limit the invention by suchdisclosure, but rather, is intended to cover all modifications andalternate constructions falling within the spirit and scope of theinvention as defined in the appended claims.

1. A spinal stabilization system, comprising: (a) first and second pedicle screws; (b) a stabilization device mounted with respect to said first and second pedicle screws; said stabilization device including (i) a first end and a second end which define an intermediate region; and (ii) an inner spring and an outer spring positioned concentrically around the inner spring; wherein the inner and outer springs are positioned at least in part within said intermediate region; wherein said stabilization device delivers a first resistive force that is based on spring resistance from both of said inner and outer springs in response to an initial range of motion associated with relative movement between said first and second pedicle screws; and wherein said stabilization device delivers a second, lesser resistive force that is based on spring resistance from only said outer spring in response to a further range of motion associated with additional relative movement between said first and second pedicle screws beyond said initial range of motion.
 2. The spinal stabilization system according to claim 1, wherein said inner and outer springs are subject to an initial preload.
 3. The spinal stabilization system according to claim 1, wherein said spinal stabilization device includes a housing positioned around the inner and outer springs in said intermediate region.
 4. The spinal stabilization system according to claim 3, wherein said housing includes first and second housing members, and wherein the relative positioning of said first and second housing members is adjustable.
 5. The spinal stabilization system according to claim 4, wherein the inner and outer springs are subject to an initial preload, and wherein adjustment of the relative positioning of said first and second housing members is effective to adjust said initial preload.
 6. The spinal stabilization system according to claim 1, wherein said initial range of motion corresponds to a neutral zone for a spine.
 7. The spinal stabilization system according to claim 1, further comprising a ball joint positioned between said first pedicle screw and said stabilization device.
 8. The spinal stabilization system according to claim 1, wherein said stabilization device includes a piston assembly, and wherein at least one of said inner and outer springs bears against said piston assembly. 